The invention is of particular interest for products which have a varying demand over time from the solar panel and the battery. An example is solar powered lighting, such as off grid street lighting.
Battery lifetime is considered a main differentiator for solar powered lighting products. Present solar powered lights use batteries to store the energy for use in the night. The present choice of battery technology is a lead (Pb) battery because of cost. Present Pb batteries have a relatively short life of 2 to 3 years. This is mainly caused by the use in a solar application. If a Pb battery is recharged immediately after discharge, and with the correct charging profile, it may enjoy a lifespan of up to 5 years. However, the solar application does not always produce enough current for a complete recharge, especially in the winter when the Pb battery is run in a partially charged condition (a so-called partial “State Of Charge”). The result is that the Pb battery has a shorter lifespan of typically 2 to 3 years, which may not be deemed sufficient.
Theoretically, lithium battery technology does have much longer life, with claims of 10 years under optimal conditions. However, also for Li-Ion batteries the outdoor solar charging conditions limit the lifespan, this time caused by the temperatures under operation. In addition, the battery is cycled (i.e. charged and discharged) every night.
In solar powered energy applications, seasonal effects cause the energy supply and demand to not be in balance. This means that batteries are fully used in winter and only partly used in summer.
The particular issues relating to outdoor solar applications will now be discussed using off grid solar powered (street) lighting technology as an example, and this will be referred to as “OSL”.
The current technology of grid-connected outdoor lighting has high installation cost, largely due to the high grid installation cost and pole installation cost. There are also high maintenance costs. For example cable repair can be required when tree roots penetrate cables.
Some of the problems of the grid connected outdoor lighting system as described above are solved by a solar powered, battery driven system, but current technology is not optimal for a number of reasons.
In warm conditions, almost all battery technologies suffer increased degradation. The mitigation is to over dimension the battery for the expected End Of Life capacity, but that costs additional money. A better way is to avoid that the battery can become too hot when charging with high(er) current. Because batteries have a limited life, they need to be replaced, and the old battery disposed of. Most battery technologies contain materials which cannot be left in the environment, such as lead or electrolytes. If non lead-based batteries are used, the need to bury them into the ground can be avoided to eliminate expensive ground works. In addition, a battery technology with longer life will reduce these costs. To reach performance requirements over the designed lifetime, batteries must be derated (operated below their maximum output), which drives up cost, volume and weight.
The depth of discharge, i.e. the depth to which charge is removed from the battery as a percentage of the full charge state, is directly related to the expected lifetime of the battery. The battery life versus Depth Of Discharge is often shown in a graphs from a IEC-896-2 cycle test. When a typical lead battery is discharged completely, the corresponding life is around 300 cycles. To achieve 600 cycles, no more than 60% of full charge may be used. For example: to reach 5 years of operational life with a charge/discharge every night, 1825 cycles are required, which is only achievable if that typical lead battery is discharged no more than 20%. This means that to reach a lifespan of 5 years requires 5 times over dimensioning based on typical Pb battery technology as used in street lighting. In fact, 5 years is not generally achieved using Pb batteries under typical outdoor, solar charging conditions.
The invention aims to enable an increase in lifespan of a battery used in a solar application, by making positive use of the need for different battery usage at different times. A battery pack for an outdoor, solar charged street lighting application is typically dimensioned for the shortest day in winter. In addition, backup capacity may be installed to provide power during several days of bad weather. The resultant energy storage requirement is often only implemented with multiple batteries (modules) put together in a series and/or parallel way.